The present invention relates to an apparatus and method for detecting an electric field.
Large power distribution systems, especially those in large metropolitan areas, are subject to many stresses, which may occasionally result in undesirable or dangerous anomalies. An infrequent, but recurrent problem in power distribution infrastructure is the presence of “stray voltages” in the system. These stray voltages present themselves when objects, such as manhole covers, gratings, street light poles, phone booths, and the like, become electrically energized, e.g., at 120 volts AC. A conductive path may be established between underground secondary network cabling and the object by physical damage to insulation that causes direct contact or by the introduction of water which serves as a conductor. These energized objects present a danger to people and animals that may come in contact with them.
Sensing the stray voltage by detecting electromagnetic radiation is impractical because the wavelength of a 60 Hz electromagnetic wave is about 5000 Km (about 3107 miles). To effectively radiate electromagnetic waves, the radiating object (manhole cover or light pole) should represent at least ¼ wavelength (about 776.75 miles) and a receiving “antenna” should be 1½ to 2 wavelengths away from the emitting source (about 6214 miles). Two wavelengths is the distance required for the electric and magnetic fields to come into time phase and space quadrature where they behave as a plane wave. A sensing system will typically be perhaps 10 ft. to 30 ft. away from the energized object, so that sensing will take place in the extreme near field where electric and magnetic fields exist in a complex temporal and spatial pattern, not as a unified electromagnetic plane wave. Therefore, the electric and magnetic fields must be considered and measured separately.
Because power distribution networks typically include many miles of buried cable carrying perhaps thousands of amperes of current, the magnetic field in any one location due to such normal load is likely to be very high. Detecting magnetic fields arising from a relatively weak stray voltage anomaly would be very difficult due to the interference from strong ambient magnetic fields arising from normal loads. Therefore, the best way to detect a stray voltage anomaly is to sense the electric field (E-field).
Because electric distribution systems are spread out over vast geographic areas, including urban, suburban and rural areas, a portable non-contacting sensor and method for sensing an electric field while moving would be desirable. For example, a sensor and method that could do so while moving at speeds of 10-25 miles per hour (about 16-40 km/hr) would be desirable.
Accordingly, there is a need for an apparatus and method for detecting an electric field that is adaptable for mobility through the electric field. For example, it is desirable to provide an apparatus and method for detecting an electric field that is adaptable for mounting on and operating from a moving vehicle to allow for drive-by inspection, which would greatly increase the rate at which potentially dangerous voltage anomalies are detected and thereby significantly improve public safety.